1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to an improvement in small size radial tires for passenger cars, middle size radial tires for truck and bus, large size radial tires for off-the-road vehicles, and super-high pressure radial tires for airplanes, construction vehicles and so on. And also, it relates to low noise radial tires which can significantly reduce noise level without damaging other tire performance such as running performance, ride comfortability against vibration, wet performance and the like.
2. Related Art Statement
In this type of the radial tire, a belt is arranged as a reinforcing layer between a tread rubber and a carcass of at least one rubberized carcass ply containing cords inclined at about 90.degree. with respect to the circumferential direction of the tire and attached at both ends to a pair of bead cores. This belt serves to increase the rigidity of the tread portion and hence to considerably enhance the cornering stability and wear resistance of the tire as well as lower fuel consumption resulted from the rolling resistance.
Therefore, steel cords are mainly used for the belt. Besides, organic fibers such as aromatic polyamide fiber, rayon fiber, polyester fiber and the like are partially used as a cord for the belt.
In case of using the steel cord, however, the ride comfortability is degraded because the bending rigidity of the tread portion becomes high. Further, since the specific gravity of the steel cord is high, the weight of the tread portion increases, which causes such drawbacks that a great centrifugal force is applied to the tread portion during high-speed running to push the end portion of the belt out toward the treading surface of the tread portion resulting in the occurrence of separation failure between belt and rubber, and that the rolling resistance becomes large due to the increase of the entire tire weight to increase the fuel consumption lately rising in importance. Further, the steel cord is apt to be corroded by water penetrating through a cut portion generated in the treading surface.
Therefore, the use of organic fiber cords, which are originally flexible and are not corroded, is expected instead of the steel cord. However, the rayon fiber cord and polyester fiber cord usually used in the belt are low in the modulus of elasticity as compared with the steel cord, so that they are insufficient in the hoop effect inherent to the belt and poor in the cornering stability and wear resistance. On the other hand, the aromatic polyamide fiber cords having a modulus of elasticity close to that of the steel cord develop the hoop effect substantially equal to that of the steel cord, but are unsatisfactory in the tire durability because such a cord is low in the fatigue resistance and adhesion property.
The use of these conventional belt cords will be described in detail below.
Among the organic fiber cords usually used in the belt, the polyester and rayon fiber cords are low in the modulus of elasticity and are used as such a fold structure that each end portion of the belt is folded radially inward back in order that it is hardly pushed out toward the treading surface by centrifugal force during the high-speed running, whereby the rigidity at the belt end portion is increased.
In such a fold structure of the belt, however, the tire-building operation becomes more complicated due to the increase of step number, and also the belt becomes particularly thicker at the folded end due to the lamination of plural belt layers, and consequently the release of heat generated during the running is less and the separation failure between cord and rubber is apt to be caused at the folded end of the belt layer. Furthermore, even when taking the fold structure, the rigidity of the belt using the polyester or rayon fiber cord is low than that using the steel cord, so that tire performances such as cornering stability, wear resistance and the like are poor. As a result, the tires comprising a belt composed of polyester or rayon fiber cords have to rank as a second-class article as compared with the steel radial tire at present.
On the other hand, the aromatic polyamide fiber cord (hereinafter referred to as aramid cord) is somewhat lower in the modulus of elasticity than the steel cord, so that there are proposed many methods of using the aramid cord for the belt, e.g. a method of piling at least two aramid cord belt layers one upon the other, a method of combining the aramid cord belt layer with steel cord belt layer, a method of folding both end portions of the aramid cord belt layer, and the like.
However, since the aramid cord is poor in the fatigue resistance, when shearing and compressive strains are applied to the treading surface of the tread portion and the belt end portion during the running, the aramid cords in the belt layer are degraded due to fatigue or the breakage of the aramid cord and the separation failure between cord and rubber at the belt end are caused by the wearing or adhesion degradation due to fretting between cords to finally cause the tire burst.
In the fold structure of the belt using the aramid cord, the compressive strain becomes large at the folded end portion, particularly at the folded side of the cord, which is apt to produce the fatigue breakage of the cord. Therefore, there is adopted a techniques for suppressing the compressive strain by increasing the thickness of the coating rubber for the belt layer to make a curvature of the folded cord portion large.
However, this techniques undesirably brings about the increase of tire weight and the promotion of heat accumulation in tire.
Moreover, the copolymer of aromatic diamine and terephthalic acid is highly oriented and crystallized in the aramid, so that the penetrability of adhesive into the aramid cord is poor. Further, only functional groups based on polyamide bond are existent in the surface of the aramid cord and the reactivity of polyamide bond is lowered by the adjacent aromatic ring, so that the reactivity of the aramid cord to the adhesive is low. Therefore, the aramid cord is considered to be fundamentally a poor adhesion cord.
Apart from the above, noises generated from automotive vehicles lately have become a great social problem, and are regulated by law, so that it is urgently needed to reduce the noise level. On the other hand, the ensuring of low noise level in the compartment passenger is required from a viewpoint of the improvement on the living environment of the automotive vehicle. In this connection, the demand for the reduction of noise level is inevitable.
Heretofore, it has been said that the improvement of tread pattern, non-uniformity of tread pattern pitch or the like is effective for the reduction of noise level in the radial tire. However, such a change of tread pattern largely affects the running performances of the tire, particularly braking performance and the like on a wet road surface, so that not only the freedom degree of tire design is largely restricted, but also it is obliged to frequently sacrifice some of the running performances for the obtention of low noise tire.
Therefore, it is desired to establish a technique capable of reducing the noise level without changing the tread pattern. However, there is found no effective technique for the reduction of noise level at present. For example, the noise level can be reduced to a certain level by applying a combination of a steel cord layer and a folded aromatic polyamide fiber cord layer (hereinafter referred to as aramid fold structure) to the belt of usual structure composed of two belt layers. The tires having such an aramid fold structure are highly valued as a low noise tire giving a good living environment even in markets, but the improving degree of noise level in such tires is 0.2-0.3 decibel at most. Thus, it is strongly desired to ensure the large reduction of noise level.
As mentioned above, the change of tread pattern for the reduction of noise level produces the degradation of braking performance on wet road surface, so that the means for reducing noise level without the change of tread pattern is attempted. In this attempt, the reduction of noise level is about 1 decibel at maximum, while it is considered that it is difficult to obtain tires satisfying the tire performances, such as running performances, ride comfortability against vibration and wet performances in addition to the reduction of noise level. Therefore, it is urgently demanded to establish a technique for the obtention of tires having the running performances, ride comfortability against vibration and wet performances equal to those of the conventional tires and reducing the noise level.